A densification mechanism to model the mechanical effect of methane hydrates in sandy sediments

Abstract

SUMMARY Recent pore‐scale observations and geomechanical investigations suggest the lack of true cohesion in methane hydrate‐bearing sediments (MHBSs) and propose that their mechanical behavior is governed by kinematic constrictions at pore‐scale. This paper presents a constitutive model for MHBS, which does not rely on physical bonding between hydrate crystals and sediment grains but on the densification effect that pore invasion with hydrate has on the sediment mechanical properties. The Hydrate‐CASM extends the critical state model Clay and Sand Model (CASM) by implementing the subloading surface model and introducing the densification mechanism. The model suggests that the decrease of the sediment available void volume during hydrate formation stiffens its structure and has a similar mechanical effect as the increase of sediment density. In particular, the model attributes stress‐strain changes observed in MHBS to the variations in sediment available void volume with hydrate saturation and its consequent effect on isotropic yield stress and swelling line slope. The model performance is examined against published experimental data from drained triaxial tests performed at different confining stress and with distinct hydrate saturation and morphology. Overall, the simulations capture the influence of hydrate saturation in both the magnitude and trend of the stiffness, shear strength, and volumetric response of synthetic MHBS. The results are validated against those obtained from previous mechanical models for MHBS that examine the same experimental data. The Hydrate‐CASM performs similarly to previous models, but its formulation only requires one hydrate‐related empirical parameter to express changes in the sediment elastic stiffness with hydrate saturation.

Thermo-Hydro-Mechanical Coupled Modeling of Methane Hydrate-Bearing Sediments: Formulation and Application

“…This volumetric distinction allows isolation of the effects of mechanical deformation from the effects of hydrate and ice fluid/solid phase transformations, both affecting the hydraulic and mechanical properties of the porous medium. We also implement the elastoplastic constitutive model Hydrate-CASM [41] to capture the effect of hydrate saturation in the sediment stress-strain response, and integrate the Peng-Robinson equation of state (EoS) [42] and the thermodynamic equations proposed by [43] to compute methane gas density and solubility, respectively.…”

“…[70,71] propose that kinematics might govern the increase in strength observed in hydrate-bearing sands. Alternatively, [41] propose that the greater strength and dilatancy observed in MHBS can be explained by densification and stiffening of the host sediment due to pore invasion by hydrate.…”

“…The constitutive model Hydrate-CASM [41] is integrated in our formulation to predict the mechanical response of MHBS. This elastoplastic model extends the formulation of the CASM model [72] by implementing the subloading surface model [73] and the densification mechanism.…”

“…The simulations are plotted against the experimental data from [58] for (a) pore-filling and (b) cementing hydrate. Adapted from [41].…”

“…The particular conditions employed in each simulation are listed in Table 7. [41]. The monitoring location shows the location of the profiles in Figure 7.…”